Template:MainarticleSigil, the "City of Doors", is located atop the Spire in the Outer Planes. It has the shape of a torus; the city itself is located on the inner surface of the ring. There is no sky, simply an all-pervasive light that waxes and wanes to create day and night. Sigil cannot be entered or exited save via portals; although this makes it quite safe from any would-be invader, it also makes it a prison of sorts for those not possessing a portal key. Thus, many call Sigil "The Bird Cage" or "The Cage". Though Sigil is commonly held to be located "at the center of the planes" (where it is positioned atop the infinitely tall Spire), some argue that this is impossible since the planes are infinite in all dimensions, and therefore there can never truly be a center to any of them, let alone all of them; thus, Sigil is of no special importance. Curiously, from the Outlands, one can see Sigil atop the supposedly infinite Spire.

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The Factions are the philosophically-derived power groups based in Sigil. Before the Faction War, the factions controlled the political climate of the city. Each of the factions is based around one particular belief system; many of the factions' beliefs make them enemies where their other goals and actions might have made them allies. Most factions are organized into covert cells for their own protection. There are fifteen factions in total, per a decree of the Lady of Pain; any additional factions emerging would be subject to her wrath.

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In 1998, TSR published Faction War, an adventure that effectively closed the book on Planescape as it was then and ending the product line. The culmination of several adventures leading up to that point, the Faction War brought an end to the factions' control of the city. Instigated by the power-hungry Duke Rowan Darkwood, factol of the Fated, in a bid to dethrone the Lady and rule Sigil himself, the war spread throughout the city before the Lady of Pain, with the aid of a group of adventurers (the players' characters), intervened.

Template:MainarticleSects are in many ways identical to the Factions, differing in that they are not based in Sigil. Sects are often highly specific to the particular planes they originate from, though historically many of the Factions were once Sects and some Sects were once Factions. A complete list of Sects is probably not possible due the infinite multitudes of the Planes.